As magnesium diboride (MgB2), superconductor discovered in Japan in 2001, has a superconducting critical temperature (Tc) of 39 K higher than that of any other metallic superconductor and is relatively easy to form into a bulk material or a tape (or wire), its physical properties and its formation into a tape and wire are widely investigated throughout the world.
A powder-in-tube (PIT) method in which a metal tube (sheath material) is filled with a raw material powder to form it into a tape or wire is known as a principal method of forming a tape and wire of a superconductor. The PIT method can be roughly classified by a difference in raw material powder into an ex-situ process utilizing a MgB2 superconducting powder itself, and an in-situ process utilizing a powder mixture of e.g. a Mg powder and a B powder and converting it into a superconductor by heat treatment after forming a tape (or wire) shape. The ex-situ process has an advantage over the in-situ process in fabricating a homogeneous tape (and wire) and being suitable for the fabrication of a long tape and wire. Moreover, while the in-situ process has no alternative but to use as a sheath material e.g. iron or a nickel alloy having no likelihood to react with the raw maternal powder at the heat treatment, the ex-situ process, which makes it possible to obtain superconducting performance without any heat treatment after forming a tape (or wire), permits a broader range of free selection for the sheath material and makes it possible to use some sheath materials with the composition suitable for its working environment, and its application is expected.